Electrophoretic display device

Abstract

An electrophoretic display device which provides high contrast images and a stable suspending fluid is described. A white marking material such as titanium dioxide is suspended in a colloidal dispersion containing black colorant such as colloidal size iron oxide particles known as ferrofluids. Image formation is achieved electrophoretically by selective application of an electric field across the imaging suspension.

What is claimed is: 
     
       1. An electrophoretic imaging process which comprises applying a voltage to a pair of electrodes spaced apart to form a cavity therebetween, at least one of said electrodes being transparent to visible light, said cavity containing an electrophoretic imaging suspension comprising (1) electrophoretic imaging particles suspended in (2) a suspending medium comprising a suspension of colloidal iron oxide particles in an electrically insulating liquid, whereby the voltage causes the electrophoretic movement of said electrophoretic particles in imagewise configuration and to visibly appear at said transparent electrode. 
     
     
       2. The process of claim 1 wherein the colloidal particles in said suspending medium have a diameter in the range of from about 50 Angstroms to about 250 Angstroms. 
     
     
       3. The process of claim 1 wherein the electrophoretic particles are white in color and the colloidal particles are black in color. 
     
     
       4. The process of claim 1 wherein the iron oxide particles have a magnetic saturation in the range of from about 100 Gauss to about 400 Gauss. 
     
     
       5. The process of claim 1 wherein the imaging suspension contains a surfactant. 
     
     
       6. The process of claim 5 wherein the surfactant is anionic. 
     
     
       7. The process of claim 6 wherein the anionic surfactant comprises bis-2-ethylhexyl sodium sulfo-succinate. 
     
     
       8. The process of claim 5 wherein the surfactant is Triton X-100. 
     
     
       9. An electrophoretic imaging process which comprises applying a voltage to a pair of electrodes spaced apart to form a cavity therebetween, at least one of said electrodes being transparent to visible light, said cavity containing an electrophoretic imaging suspension comprising electrophoretic zinc oxide particles suspended in a suspended medium comprising colloidal sized particles of iron oxide suspended in an electrically insulating liquid, said imaging suspension also containing an anionic surfactant whereby the voltage causes electrophoretic movement of said electrophoretic particles in imagewise configuration and to visible appear at said transparent electrode.